Focusing means for cathode ray tubes

ABSTRACT

A cathode ray type tube wherein an electron gun is operatively disposed for directing an electron beam toward an axially spaced target surface, the gun being provided with means for producing a sharply focused beam having a focal spot suitable for scanning the target surface in a writing mode and alternatively producing a less sharply focused beam having a higher electron density and a larger focal spot suitable for scanning and uniformly charging the target surface.

United States Patent Spencer [45] Sept. 9, 1975 [54] FOCUSING MEANS FORCATHODE RAY 2,942,128 6/[960 Johnson .i 313/82 R TUBES 3,320,457 5/l967Burdick ct al, 3 l 3/82 BF 3,354,335 ll/l967 Corpcw .l 313/86 KM X [75]Inventor: Gordon R. Spencer, Westwood,

Ma55- Primary Examiner-Robert Segal [73] Assignee: Raytheon Company,Lexingtonq Attorney, Agent, or Firm-.lohn T. Meaney; Harold A.

Massv Murphy; Joseph D. Pannone [22] Filed: Aug. 27, 1973 [57] ABSTRACT[2l] APPL N0- 391,916 A cathode ray type tube wherein an electron gun isoperatively disposed for directing an electron beam [52] U.s. C1313/449; 313/391 toward a axially spaced target Suffaehe gu being [5l]lm. CL2 HOU 29/02; HOU 31,58 provided with means for producing a sharplyfocused [58] Field of Search I 313/71, 68 D` 86 82 R1 beam having afocal spot suitable for scanning the tar- 33/82 BF get surface in awriting mode and alternatively producing a less sharply focused beamhaving a higher [56] References Cited electron density and a largerfocal spot suitable for scanning and uniformly charging the targetsurface.

2 Claims, 5 Drawing Figures PATENTEU SEP 9 i975 .air 1 of 2 FOCUSINGMEANS FOR CATHODE RAY TUBES BACKGROUND OF THE INVENTION This inventionis related generally to electron tubes of the cathode ray type and isconcerned more particularly with an electron gun having means foruniformly charging a raster area of a target surface.

An electron tube of cathode ray type generally comprises an evacuatedenvelope wherein an electron gun disposed adjacent one end of theenvelope emits an electron beam which scans a target surface of anelectrode disposed adjacent the other end of the envelope. Tubes of thedescribed type may include orthicons, vidicons, storage tubes and thelike which may be operated in different modes, such as priming, writing,reading and erasing, for examples.

The electron gun usually comprises an electron emitting cathode and acoaxially aligned series of spaced grid, anode, and focusing elements.Aberrations introduced by these elements generally are minimized byrestricting the diameter of the beam such that it includes only an axialcentral portion of the beam. This may be achieved by an axially alignedanode having a transverse portion wherein an aperture is centrallydisposed. Thus` a central axial portion of the beam passes through theaperture, and a surrounding annular portion of the beam impinges on thetransverse portion of the beam limiting anode. Accordingly, a sharplyfocused beam is provided for scanning the target surface, but theelectron current of the beam is reduced considerably.

The sharply focused scanning beam is advantageously used when highresolution is required, as in the writing and reading modes, forexamples. However, when the target is being uniformly' charged to adesired potential, as in the priming and erasing modes, for example, theentire electron beam generally is preferred for scanning the targetsurface. Consequently, preceding the beam limiting anode member in theelectron gun, there may be disposed an enhancement electrode whichdirects substantially all thc electrons in the beam through the aperturein the anode when desired. Due to the necessarily short focal distanceinvolved, a large portion of these electrons follow divergingtrajectories after passing through the aperture. Accordingly, theenhanced beam produces at the target surface an excessively largescanning spot, which may be as large as 2U percent of the targetsurface` for example. This increase in the scanning spot size generallyis tolerated, because the associated increase in electron currentprovides means for charging the target surface in a shorter timeinterval than required when using the sharply focused beam.

However, when the excessively large scanning spot is used for chargingthe target surface, it generally is found that the effective raster areaof the surface is Charged non-uniformly and a surrounding marginal areais charged unnecessarily. Thus, non-uniform priming of the targetsurface causes inaccuracies to occur in subsequent writing and readingmodes. Also, nonuniform erasing of the target surface results inresidual stored data being carried over into a subsequent frame ofstored information. As a result, confusing and erroneous information isobtained from the tube during subsequent reading modes.

Therefore, it is essential that electron tubes of the cathode ray typebe provided with means for controlling the size of the scanning spotwhen the entire electron beam is being used for uniformly charging atarget surface.

SUMMARY OF THE INVENTION Accordingly, this invention provides a cathoderay type tube wherein an electron gun is operatively disposed fordirecting an electron beam toward an axially spaced target surface, thegun including means for producing a beam having a focal spot suitablefor scanning the target surface in a writing mode and alternativelyproducing a beam having a higher electron current and a focal spotsuitable for scanning and uniformly charging the target surface.

The electron gun comprises a beam generating means disposed adjacent oneend of the gun for forming an electron beam directed toward the otherend of the gun, and beam control means disposed adjacent the other endof the gun for varying the cross-sectional Size and focusing of thebeam. The beam generating means includes a series of beam formingelements axially aligned with an electron emitting cathode, and the beamcontrol means includes a plurality of beam size determining means, eachassociated with a respective focusing means. One of the beam sizedetermining means may include a beam limiting electrode axially alignedwith the series of beam forming elements, and the associated focusingmeans may include a subsequent axially aligned focusing electrode. Thebeam limiting electrode may comprise an apertured anode which permitsthe passage of only a central axial portion of the beam formed by theseries of beam forming elements. The associated focusing electrode ismaintained at an appropriate potential for focusing the electrons in thelimited beam onto a focal spot which is suitable for scanning the targetsurface in a writing mode of operation. Another one ofthe beam sizedetermining means may comprise an enhancement electrode disposed inpreceding axial alignment with the apertured anode. The enhancementelectrode may be pulsed to a potential suitable for directingsubstantially the entire electron beam formed by the series of elementsthrough the apertured anode. The focusing means associated with theenhancement electrode may comprise a focusing electrode disposed insubsequent axial alignment with the focusing electrode associated withthe apertured anode. The enhancement focusing electrode is pulsed to thesame potential as the enhancement electrode and, consequently, may beelectrically coupled directly to the enhancement electrode. When theenhancement focusing electrode is maintained at the potential of theenhancement electrode, it is found that the greater number of electronsin the enhanced beam passing through the apertured anode are focusedonto a focal spot of suitable size for scanning and uniformly chargingthe target surface.

The preferred embodiment, shown herein, comprises a storage tube havingan evacuated envelope wherein a mesh-like storage electrode is disposedbetween a collector electrode and an axially spaced electron gun. Thestorage electrode is provided with a dielectric target surface which isscanned by an electron beam emanating from the gun during operation ofthe tube. By applying suitable voltage to the storage electrode andregulating the electron beam accordingly, the storage tube may beoperated in a well-known sequence of priming` writing` reading anderasing modes.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thisinvention, the following, more detailed, description makes reference tothe accompanying drawings wherein:

FIG. l is an axial view. partly in section, of a preferred embodiment ofthis invention-` FlG. 2 is a schematic axial view of a prior art type ofelectron gun',

FIG. 3 is a diagrammatic view of a target surface scanned by an enhancedelectron beam emanating from the gun shown in FIG. 2;

FIG. 4 is a schematic axial view of the electron gun shown in FIG. l;and

FIG. 5 is a diagrammatic view of a target surface scanned by an enhancedelectron beam emanating from the gun shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawingswherein like characters of reference designate like parts` FIG. l showsa storage tube l0 comprising a tubular envelope l2 which preferably ismade of dielectric material, such as glass, for cxample, and is providedwith a neck portion 14. The neck portion I4 tapers outwardly at one endto merge with an end of a relatively large diameter portion 16 ofenvelope I2. Pcripherally sealed to the other end of portion 16 is atransversely disposed end plate 18 which closes one end of the envelopeI2. The other end of neck portion I4 is peripherally sealed to atransversely disposed disk 20 which closes the other end of envelope l2.The end disk 20 is provided with an evacuation tubing 22 which may bemade of glass, for example, and which is sealed-off after processing ofthe tube 10 is completed. Extending axially through the end disk 20, ina vacuum-tight manner, is a plurality of insulatingly spaced terminalpins 23. The pins 23 provide means for applying voltages to respectiveelectrodes of an electron gun 24 which is axially disposed within theneck portion 14 of envelope l2.

The electron gun 24 includes an electron emitting cathode 26 whichconstitutes the generating end of the gun and is disposed adjacent theend disk 20. The cathode 26 may be directly heated, or may be indirectlyheated, as by means of filament 28, for example. Cathode 26 is axiallydisposed in spaced relationship within a first grid cup 30 which has anaperture 32 centrally disposed in its closed end. Axially spaced fromthe closed end of grid cup 30 is a similar closed end of a second gridcup 34 having a centrally disposed aperture 36 which is aligned with theaperture 32. The first and second grid cups, 30 and 34, respectively,regulate the flow of electrons from the cathode 26 and aid in formingthe emitted electrons into a beam. The open end of grid cup 34 isdisposed in spaced relationship with one end of a coaxially disposed,hollow cylinder 38 which serves as the first anode of gun 24. The firstanode cylinder 38 is maintained at a suitable potential for acceleratingthe electrons emerging from second grid cup 34 and further forming theminto a beam.

The opposing end of anode cylinder 38 is disposed in spaced relationshipwith one end of a coaxial hollow cylinder 40 which functions as anenhancement electrode when desired. Axially spaced from the opposing endof the enhancement electrode is an open end of a coaxially disposedsleeve 42 having therein a transverse wall 44 which is provided with acentral aperture 46. The sleeve 42 functions as a beam limiting anodewhich permits only an axial central portion of the electron beam to passthrough the aperture 46 when a sharply focused beam is required. As aresult, a surrounding annular portion of the beam impinges on thetransverse wall 44 and is absorbed by the beam limiting anode member 42.In this mode of operation, the enhancement electrode 40 may bemaintained at a suitable potential for functioning as an electronaccelerating anode. However, when use of the entire electron beam isdesired, the enhancement electrode 40 is pulsed to a suitable potentialfor directing substantially all the electrons in the beam through theaperture 46 of the beam limiting anode 42. Due to the necessarily closespacing of the enhancement electrode 40 to the aperture 46, a largeportion of the electrons in the enhanced beam follow mutually' divergingtrajectories after passing through the aperture.

The opposing end portion of beam limiting anode 42 may taper inwardly toform a reduced diameter end, and is disposed in spaced relationship withone end of a coaxial hollow cylinder 48. The cylinder 48 comprises afocusing electrode which is maintained at a suitable potential forsharply focusing the central axial portion of the electron beam definedby the aperture 46. As such, the focusing electrode 48 is inadequate forfocusing the diverging electrons in the enhanced beam. Preferably,focusing of the enhanced beam is achieved in a manner which does notrequire radical structural changes or redesign of external circuitryconnected to the electrodes of the gun 24.

The other end of focusing electrode 48 is disposed in spacedrelationship with a preferably inward tapering end portion ofa coaxiallydisposed, hollow cylinder 50. The cylinder 50 constitutes a second anodecylinder of the gun 24 and has an opposing end disposed in spacedrelationship with a coaxial hollow cylinder S2. The cylinder 52comprises an enhancement focusing electrode which functions as an anodewhen only the axial central portion of the beam is passing through theaperture 46. However, when substantially the entire electron beam isdirected through the aperture 46, the enhancement focusing electrode 52is pulsed to the same potential as the enhancement 40. Axially spacedfrom the opposing end of the enhancement focusing electrode 52 is an endof a hollow coaxial cylinder 54 which constitutes a third anode cylinderof the gun 24. By positioning the enhancement focusing electrode 52between the two anode cylinders, 50 and 54, respectively, the electrode52 appears electrostatically` when functioning as an anode, to be anextension of the anode cylinder S0 and 54. Thus, the enhancementfocusing electrode 52 is readily introduced into the structure of gun 24and does not require a redesign of external circuitry. Since theenhancement focusing electrode 52 is pulsed to the same potential as theenhancement electrode 40, it may conveniently be connected electricallythereto, by conventional means, within the envelope l2. The enhancementfocusing electrode S2 is suitably positioned with respect to aperture 46in beam limiting anode 42 to produce the desired focusing effect on theelectrons in the enhanced beam.

The described electrodes of the electron guri 24 may be supported ininsulating spaced relationship with one another by suitable attachment,to a plurality of spaced side rods, such as 56 and 58 for examples. Theside rods 56 and 58, respectively, extend axially within the neckportion 14 of envelope 12 and are preferably made of dielectricmaterial, such as glass, for example. Disposed in spaced coaxialrelationship with the third anode 54 is a hollow cylinder 60 whichconstitutes the fourth anode cylinder of gun 24. Cylinder 60 maycomprise a continuous band of conductive material, such as graphite, forexample, which is adherringly deposited by conventional means on theinner surface of the neck portion 14. The third anode 54 is electricallycoupled to the fourth anode 60 by suitable means, such as radial flange62 extending outwardly from the third anode cylinder S4 and supporting aplurality of resilient tabs 64 in electrical contact with an adjacentend portion of cylinder 60, for example. The opposing end portion offourth anode cylinder 60 may extend onto the outwardly tapering wall ofneck portion 14 and constitutes the exit end of electron gun 24.

The gun 24 is eoaxially aligned with the axis of tube and projects abeam of electrons, as described, into the large diameter portion 16 ofenvelope 12. The electron beam may be deflected in a predeterminedmanner by suitable means, such as external deflection yoke 66, forexample, which encircles the fourth anode cylinder 60 within neckportion 14. Within the large diameter portion 16 of envelope 12, theelectron beam passes through a pair of axially spaced, hollow cylinderswhich constitute first and second collimating electrodes, 68 and 70,respectively. Each of the electrodes 68 and 70, respectively, maycomprise a continuous band of conductive material, such as graphite, forexample, which is adherringly deposited by conventional means 0n theinner surface of large diameter portion 16. The collimating electrodes68 and 70 are electrically connected to respective terminal buttons 69and 7l which extend in a vacuum-tight manner through the wall ofenvelope l2. The terminal buttons 69 and 71 provide means formaintaining the collimating electrodes 68 and 70, respectively, atsuitable potentials for aligning the electron beam with the axialcenterline of tube l0.

Axially spaced from the second collimating electrode 70 is atransversely disposed decelerator electrode 72 which generally comprisesa fine mesh screen stretched across the opening of a conductive supportring. The decelerator electrode 72 is disposed in coaxial alignment withthe axial centerline of tube l0, and is insulatingly secured to aparallel storage electrode 74. The storage electrode 74 also generallycomprises a fine mesh screen stretched across the opening Of aconductive support ring, but it is provided with a target surface 76 bycoating one surface with a film of dielectric material, such as glass,for example. The decelerator electrode 72 and the storage electrode 74are electrically connected to respective terminal buttons 73 and 75which extend through the wall of envelope l2 in a vacuum-tight manner.The storage electrode 74 is insulatingly coupled to a parallel collectorelectrode 78 which may comprise a thin conductive plate made oftitanium, for example. The collector electrode 78 is electricallyconnected to a supporting terminal member 79 which may extend axiallythrough the end plate 18 in a vacuum-tight manner.

ln operation, the deflection yoke 66 produces a varying magnetic fieldwhich causes the electron beam emanating from gun 24 to scan a desiredraster area of the dielectric target surface 76, in a well-known manner.The decelerator electrode 72 generally is maintained at a lower positivepotential than the exit end of gun 24 for the purpose of establishing aretardation field which decelerates the beamed electrons as theyapproach the target surface 76. As a result, the first and secondcollimating electrodes, 68 and 70, respectively, are enabled to directthe electrons electrostatically into a perpendicular approach to thetarget surface 76, while the beam is scanning the raster area of thetarget surface. ln this manner, the storage tube l0 may be operated in awell-known sequence of priming, writing, reading and erasing modes.

Briefly, in the priming mode, the scanning electron beam depositselectrons on the raster area of the dielectric target surface 76 tocharge it uniformly to the potential of the cathode 26 in gun 24. Then,for the writing mode, the potential of target surface 76 ge nerally isincreased to a considerably higher positive value, such that thescanning electron beam causes localized emission of secondary electronsfrom discrete elements of the raster area. However, in the writing mode,the scanning beam is modulated by an information signal which usually isapplied to the control grid 30 of gun 24. Consequently, the signal isstored on the raster area of target surface 76 in a varying pattern ofpositively charged elements which correspond to modulations introducedin the scanning beam by the information signal, Subsequently, for thereading mode, the potential of target surface 76 generally is decreasedto a value slightly below cathode potential whereby the elements leastcharged in the writing mode may repel electrons from the scanning beam.Thus, in the reading mode varying amounts of electrons from the scanningbeam are allowed to pass through respective apertures in the mesh-likestorage electrode 74 depending on the charged elements adjacent theapertures. Accordingly, the collector electrode 80 receives asequentially varying electron current which constitutes a nondestructivereading of the signal stored on the raster area of target surface 76.This signal may be erased from the raster area by increasing thepotential of target surface 76 to a relatively high positive value withrespect to the cathode 26. As a result, the scanning beam produces asaturation emission of secondary electrons from successive discreteelements of the raster area thereby charging it uniformly to a higherpositive potential with respect to cathode 26.

Thus, it may be seen that a sharply focused electron beam is desirablefor the writing and reading modes where resolution is required in thestoring and reading of information data. However, in the priming anderasing modes where fast uniform charging of the target surface isdesired, it is more advantageous to utilize the full beam current.Consequently, as shown in FIG. 2, a prior art electron gun for a similartube of the cathode ray type may comprise a eoaxially aligned series ofspaced electrodes including a cathode 26a, respective first and secondgrid electrodes 30a and 34a, first anode cylinder 38a, enhancementelectrode 40a, beam limiting anode 42a having a transverse portion 44awherein an aperture 46a is centrally disposed, focusing electrode 48a,and second anode cylinder 50a. The anodes 38a, 42a and 50a mayconveniently be connected electrically to one another internally of thetube envelope, since they are generally maintained at the sameelectrical potential.

In order to minimize aberrations, the electron beam produced by thedescribed prior art gun is reduced diametrically to include only anaxial central portion of the beam. Consequently, the desired axialcentral portion of the beam is permitted to pass through the aperture46a in beam limiting anode 42a and the surrounding annular portion ofthe beam impinges on the transverse wall portion 44a. ln this manner, asharply focused electron beam is obtained for the reading and writingmodes, but the beam current is reduced considerably. When full beamcurrent is required for the priming and erasing modes, a suitablepotential is applied to the enhancement electrode 40u for focusingsubstantially the entire electron beam through the aperture 46a in beamlimiting anode 42a. However, a large portion of the electrons passingthrough the aperture 46a follow mutually diverging paths which are notadequately corrected by the focusing electrode 48a. Consequently, asshown in FIG. 3, the enhanced beam produces at the target surface 76a anexcessively large scanning spot 80a. As a result, only a central portion84a of a desired raster area 82a is charged uniformly. A surroundingarea 86a ofthe raster area 82u is insufficiently charged, and an outermarginal area 88a of the target surface 76u is charged unnecessarily.

As shown in FIG. 4, the electron gun 24 of this invention is providedwith an enhancement focusing electrode 52 which is connectedelectrically to the cnhancement electrode 40 internally of the envelopel2. Consequently, additional external connection or associated changesin external circuitry are not required. Also, when a suitable pulsevoltage is applied to the enhancement electrode 40 for focusingsubstantially the entire electron beam through aperture 46 the samepulse voltage is applied simultaneously to enhancement focusingelectrode 52. As a result, the enhancement focusing-electrode 52 isestablished at an optimum potential for refocusing the enhanced beamonto a desired focal spot 80, as shown in FIG. 5. The focal spot 80 isrelatively smaller than the excessively large scanning spot 80u and`therefore. is more suitable for scanning and uniformly charging awell-defined raster area 82 of target surface 76, as required for thepriming and erasing modes, for examples. However, the focal spot 80,preferably, is slightly larger than the scanning spot utilized for thewriting mode in order to ensure the erasure of line structure producedduring the previous writing modes.

From the foregoing, it will be apparent that all of the objectives ofthis invention have been achieved by the structure shown and described.lt also will be apparent, however, that various changes may be made bythose skilled in the art without departing from the spirit of theinvention as expressed in the appended claims. lt is to be understood,therefore, that all matter shown and described is to be interpreted asillustrative and not in a limiting sense.

l claim:

l. An electron gun comprising:

an electron emitting cathode disposed adjacent one end of the gun fordirecting a beam of electrons toward an opposing output end of the gun;

a beam limiting anode cylinder operatively aligned with the cathode andhaving therein a transverse wall provided with a central aperture forpermitting a limited electron beam therethrough;

a first focusing sleeve electrode operatively disposed in alignmentbetween the cathode and the beam limiting anode for intermittentlyfocusing an enhanced electron beam through the aperture;

a second focusing sleeve electrode operatively disposed between the beamlimiting anode and the output end of the gun, and positioned apredetermined axial distance from said wall member for focusing saidlimited electron beam;

a third focusing sleeve electrode operatively disposed between thesecond focusing sleeve electrode and the output end of the gun, andpositioned a predetermined axial distance from said wall member forfocusing said enhanced electron beam, the third focusing electrode beingelectrically coupled to the first focusing electrode by aninterconnecting conductive member;

an intermediate anode cylinder operatively disposed in alignment betweenthe second and third focusing electrode', and

an output anode cylinder operatively aligned with the third focusingelectrode and disposed adjacent the output end of the gun.

2. An electron tube of the cathode ray type comprising:

an evacuated envelope;

an electrode having a target surface transversely disposed within theenvelope; and

an electron gun operatively disposed within the envelope for directingtoward the target surface a sharply focused electron beam and,alternatively, a less sharply focused, higher density electron beam,each beam having a focal spot suitable for scanning thc target surface,the gun including;

an electron emitting cathode disposed adjacent one end of the gun fordirecting a beam of electrons toward on opposing output end of the gun;

a beam limiting anode cylinder operatively aligned with the cathode andhaving therein a transverse wall member provided with a central aperturefor permitting a limited electron beam therethrough;

a first focusing sleeve electrode operatively disposed in alignmentbetween the cathode and the beam limiting anode, and spaced a sufficientaxial distance from the wall member for intermittently focusing a higherdensity electron beam through said aperture; second focusing sleeveelectrode operatively disposed between the beam limiting anode and theoutput end of the gun, and spaced a sufficient axial distance from thewall member for sharply focusing the limited electron beam onto saidtarget surface; a third focusing sleeve electrode operatively disposedbetween the second focusing electrode and the output end of the gun, andspaced a sufficient axial distance from the wall member for focusing thehigher density electron beam less sharply onto said target surface, thethird focusing electrode being electrically coupled directly to thefirst focusing electrode by an interconnecting conductive member withinthe envelope; an intermediate anode cylinder operatively disposed inalignment between the second and third focusing electrodes; and anoutput anode cylinder operatively aligned with the third focusingelectrode and disposed adjacent the output end of the gun.

1. An electron gun comprising: an electron emitting cathode disposedadjacent one end of the gun for directing a beam of electrons toward anopposing output end of the gun; a beam limiting anode cylinderoperatively aligned with the cathode and having therein a transversewall provided with a central aperture for permitting a limited electronbeam therethrough; a first focusing sleeve electrode operativelydisposed in alignment between the cathode and the beam limiting anodefor intermittently focusing an enhanced electron beam through theaperture; a second focusing sleeve electrode operatively disposedbetween the beam limiting anode and the output end of the gun, andpositioned a predetermined axial distance from said wall member forfocusing said limited electron beam; a third focusing sleeve electrodeoperatively disposed between the second focusing sleeve electrode andthe output end of the gun, and positioned a predetermined axial distancefrom said wall member for focusing said enhanced electron beam, thethird focusing electrode being electrically coupled to the firstfocusing electrode by an interconnecting conductive member; anintermediate anode cylinder operatively disposed in alignment betweenthe second and third focusing electrode; and an output anode cylinderoperatively aligned with the third focusing electrode and disposedadjacent the output end of the gun.
 2. An electron tube of the cathoderay type comprising: an evacuated envelope; an electrode having a targetsurface transversely disposed within the envelope; and an electron gunoperatively disposed within the envelope for directing toward the targetsurface a sharply focused electron beam and, alternatively, a lesssharply focused, higher density electron beam, each beam having a focalspot suitable for scanning the target surface, the gun including: anelectron emitting cathode disposed adjacent one end of the gun fordirecting a beam of electrons toward on opposing output end of the gun;a beam limiting anode cylinder operatively aligned with the cathode andhaving therein a transverse wall member provided with a central aperturefor permitting a limited electron beam therethrough; a first focusingsleeve electrode operatively disposed in alignment between the cathodeand the beam limiting anode, and spaced a sufficient axial distance fromthe wall member for intermittently focusing a higher density electronbeam through said aperture; a second focusing sleeve electrodeoperatively disposed between the beam limiting anode and the output endof the gun, and spaced a sufficient axial distance from the wall memberfor sharply focusing the limited electron beam onto said target surface;a third focusing sleeve electrode operatively disposed between thesecond focusing electrode and the output end of the gun, and spaced asufficient axial distance from the wall member for focusing the higherdensity electron beam less sharply onto said target surface, the thirdfocusing electrode being electrically coupled directly to the firstfocusing electrode by an interconnecting conductive member within theenvelope; an intermediate anode cylinder operatively disposed inalignment between the second and third focusing electrOdes; and anoutput anode cylinder operatively aligned with the third focusingelectrode and disposed adjacent the output end of the gun.